Team Rocket is one of Pokémon's antagonist teams, along with Team Magma and Team Aqua.
Their leader was Giovanni. He also gives you a Silph Scope in the first generation of Pokémon video games, after beating him.
Team Rocket in the anime had two main characters - Jesse and James, along with the talking Meowth, although in other editions of the anime, there were other people featured.
They generally have Pokémon of the Poison type, such as Golbat, Ekans and Koffing.